A polarizing filter on your camera is probably the most important filter you can use. It removes polarized light from the image, thus reducing reflections and glare, while at the same time increasing colour saturation – especially that of a blue sky. A circular polarizing filter works by rotating an outer ring to vary the amount of polarization that is being filtered. This effect is difficult to mimic on the computer.
Visible light from the sun travels in a straight line as a wave that is oscillating in all directions, such as up and down, and from side to side. When that light is reflected off an object, it is the reflected wavelength of the light that determines the colour of the object. The rest of the colours are absorbed by the object.
For example, a pure blue-coloured object reflects only blue light and absorbs the red, orange, yellow, green, indigo, and violet parts of the light. Likewise, green leaves on trees only reflect green and some yellow, whilst absorbing the other colours.
If the light being reflected or scattered travels in only one direction – the polarized one – it will cause glaring and reduce the colour intensity of the reflected surface. So, by using a special filter to remove this polarized light, the colour intensity will be restored.
A polarizing filter has a layer of Polaroid sandwiched between two glass plates. In a circular polarizing filter, as the front plate is rotated, the angle of polarization and thus the amount of polarized light that passes through the filter changes. This allows precise control of the degree of polarized light that is to be removed.
Now, when sunlight encounters atoms in the Earth’s atmosphere, which are mostly of nitrogen and oxygen, the light scatters. Blue light is scattered more than red light, which is why a clear sky during the day appears blue in colour.
The electric field of scattered light tends to oscillate in one direction. So, if the photographer is looking perpendicularly to the direction from which the light is travelling, it will be polarized because the energy goes one way.
By applying a Polaroid filter, the energy of the light along one axis can only pass through, hence why the filtering effect works best when the sun is at right angles to the direction the photo is being taken, and the sky will appear to be darker blue in colour.
If the sun is in front or behind the photographer, the polarizing filter will make no difference because the axis of oscillation will not be filtered by the Polaroid, and thus no effect will be seen.
There are two kinds of polarizing filter available to photographers – linear and circular, but only the circular type will work properly with digital cameras. This is because the linear type affects metering accuracy as autofocus cameras already polarize some light inside the camera and a false meter reading will be obtained.
Circular polarizing filters are made with a wave-retardation plate, exactly a quarter of a wavelength in thickness, which lets the light to passing into the metering system appear unpolarized. However, if you do have a linear polarizer, simply take a meter reading before attaching the filter, and then increase exposure time by a couple of stops. That should do the trick. With film cameras it makes no difference which type of filter you use.
About the Author
Chris Smith gives photography tips in his ezine that you can get every week for no cost. He has also put together a complementary report for you called ‘How To Master White Balance’, which helps you solve any white balance problems. To download it instantly and to obtain the ezine please visit: photography-expert.com.
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